The present invention relates to a heat-resistant cast steel suitable for exhaust equipment members, etc. for automobile engines, and more particularly to a heat-resistant cast steel having excellent durability such as a thermal fatigue resistance, thermal deformation resistance and oxidation resistance, castability and machinability, which can be produced at a low cost, and an exhaust equipment member made of such a heat-resistant cast steel.
Conventional heat-resistant cast iron and heat-resistant cast steel have compositions shown in Table 1 as Comparative Examples. Exhaust equipment members such as exhaust manifolds, turbine housings, etc. for automobiles are exposed to extremely severe conditions at high temperatures. Therefore, as materials for such exhaust equipment members, heat-resistant cast iron such as high-Si spheroidal graphite cast iron, NI-RESIST cast iron (Ni-Cr-Cu austenite cast iron), etc. shown in Table 1, heat-resistant, ferritic cast steel disclosed in JP-A-2-175841 (U.S. Pat. No. 5,106,578) and exceptionally expensive heat-resistant, high-alloy cast steel such as austenite cast steel, etc. have been employed.
Among these conventional heat-resistant cast iron and heat-resistant cast steel, for instance, high-Si spheroidal graphite cast iron and NI-RESIST cast iron are relatively good in castability, but they are poor in durability such as a thermal fatigue resistance and an oxidation resistance. Accordingly, they cannot be used for members which may be subjected to such a high temperature as 900.degree. C. or higher. Heat-resistant ferritic cast steel disclosed in JP-A-2-175841 is good in thermal fatigue resistance but poor in thermal deformation resistance. Also, heat-resistant, high-alloy cast steel such as heat-resistant austenite cast steel, etc. is excellent in a high-temperature strength and thermal deformation resistance at 900.degree. C. or higher, but the high-alloy east steel is poor in a thermal fatigue resistance due to a large thermal expansion coefficient. Further, because of poor castability, the high-alloy cast steel is likely to suffer from casting defects such as shrinkage cavities and poor fluidity in casting process. In addition, because of poor machinability, the production of parts from the high-alloy cast steel is not efficient. Besides the above cast iron and cast steel, ferritic cast stainless steel has been known. However, usual ferritic cast stainless steel shows poor ductility at room temperature when its high-temperature durability is improved. Accordingly, ferritic cast stainless steel cannot be used for members which are subjected to mechanical impact, etc.